Recently, some high-end vehicles have been equipped with vehicle-mounted display systems. For the vehicle-mounted display system, usually a projector is used to project a video image onto a separate projection screen or a front windshield of the vehicle, and the projected video image may be viewed by a driver when reflected by the projection screen or the front windshield. However, the projection screen or the front windshield is generally made of common glass and cannot shield light beams completely, i.e., the projected video image may be viewed by a person outside the vehicle. As a result, it is impossible to ensure the privacy of the video image, and it is difficult to meet the requirements of high-end business persons.